This invention relates to a method of nondestructive testing of steam turbine rotors and more particularly to a method of testing rotors to make a determination of whether the material of the rotor has become embrittled to a point where the rotor should be retired.
Steam rotors are subject to progressive embrittlement during operation at elevated temperatures as a result of the migration of impurities, such as antimony and arsenic, to the grain boundaries in the steel. These impurities form compounds, which weaken the grain boundaries creating zones of brittleness, which become subject to crack development. The progressive embrittlement could eventually lead to failure of the rotor, which can occur in a massive fracture during operation with catastrophic results, including tremendous property loss and loss of life. In the past there have been a number of major fractures of steam turbine rotors, which fractures can be attributed to this phenomenon of progressive embrittlement. At the present time there is no satisfactory way of testing the rotors for this condition of embrittlement. The inspection systems presently employed are sonic bore testing, in which ultrasonic waves are applied to the rotor bore, visual inspection for cracks, and taking test coupons from some location on the rotor and destructively testing the rotor material thus obtained. The problem is that neither of the first two methods test for the condition of embrittlement. The sonic bore testing and visual inspection will indicate the presence of cracks or other abnormalities in the steel structure, but embrittlement can occur without these abnormalities and the abnormalities can and do exist in rotors, which have not yet become excessively embrittled. Test coupons will give an indication of whether the part of rotor from which they are taken has become embrittled, but they should be taken only from places on the rotor remote from where embrittlement is likely to occur and thus fail to provide a reliable test for embrittlement. As a result there is a great deal of uncertainty as to when a steam turbine rotor should be retired. Because of the cost of the rotors, there is a considerable reluctance upon the part of the users to scrap a rotor, even when cracks are detected in the rotor. One rationalization or procedure employed by the turbine users to justify keeping a rotor in operation after a rotor flaw is detected is to keep track of the number of cold starts of the rotor after the flaw has been detected and make the decision whether or not to retire the rotor only after a number of cold starts have been experienced by the rotor. However, this procedure assumes that the eventual failure of the rotor would be caused by low cycle fatigue rather than embrittlement, as the progressive embrittlement of the rotor is not substantially affected by the number of cold starts of the rotor. The present uncertainty as to when is the proper time for a rotor to be retired has led to the situation in which the users of the rotors are continuing to operate their old rotors, even though the manufacturers of these rotors recommend that they be retired.